Rangin, C, Silver, E., von Breymann, M. T., et al., 1990 Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 124 7. HYDROGRAPHY OF THE SULU SEA1 Andrea Frische2 and Detlef Quadfasel2 INTRODUCTION 15°N Like most other deep basins in the eastern Indonesian archi- pelago, the deep Sulu Basin is isolated from the neighboring ba- sins by surrounding shallower topography. Therefore, its hydro- graphic and dynamic characteristics are representative for the deep basins of the archipelago. While the near-surface circula- tion is mainly governed by the seasonally reversing monsoon winds, the deep circulation is forced by an inflow of intermedi- ate water from the South China Sea through the Mindoro Strait (sill depth 420 m), which supplies the intermediate and bottom waters of the Sulu Sea (Wyrtki, 1961). Below 1000 m tempera- tures and salinities in the Sulu Sea are remarkably homogeneous (0 = 9.8°-10.0°C, S = 34.48). Tracer data (Broecker et al., 1986) suggest renewal times of 50-100 yr. In the past only few hydrographic data were collected in the Sulu Sea. This sparse data set did not allow the delineation of a detailed pattern of deep circulation and water mass renewal. For this reason a closely spaced hydrographic section from the South China Sea to the central Sulu Sea was occupied during cruise SO 58 (Fig. 1, Table 1). The main objectives of this program were: 1. to map the structure and depth of the core of the inflow- ing intermediate water from the South China Sea in detail; 2. to estimate the mixing of this water with the ambient wa- ters in the Mindoro Strait and the slope region in the northern Sulu Basin; and 3. to investigate the influence of geothermal heating on the deep waters and the vertical circulation of the Sulu Sea. Apart from conductivity, temperature, and pressure (CTD) casts, water samples were taken at up to 12 depth levels and ana- 117°E 120° 124C lyzed for salinity, dissolved oxygen, and nutrient contents. Here we present preliminary results from this investigation, Figure 1. Location of hydrographic stations in the South China Sea and based upon the hydrographic bottle data and the CTD tempera- Sulu Sea during cruise SO 58, 28 August to 17 September, 1988. Depth tures. contours are in fathoms. DATA COLLECTION CTD, the remaining two casts were recorded on the VAX com- Temperature and salinity profiles were obtained with two puter of the Sonne. ME-Multisonde CTDs providing data with a resolution of 0.2 The oxygen samples were analyzed on board with a Hydro- dbar for pressure, 0.0015°C for temperature, and 0.002 mS/cm bios tritration stand using the Winkler method. Reproducibility for conductivity. The CTDs were supplemented by a Hydrobios on duplicate samples was better than 0.07 mg/L. The salinity rosette sampler equipped with twelve 5-L bottles. Two bottles samples were analyzed after the cruise, at the Institut für Meeres- carried protected and unprotected reversing thermometers. The kunde in Hamburg using a Guildine Autosal Salinometer. Even raw data from the CTD were stored directly on digital magnetic with a storage time of 3 months, accuracy was better than tape at a rate of 8 cycles per second. In addition, every eighth ± 0.003‰. The nutrient samples were fixed with mercuric chlo- cycle was logged onto a personal computer to serve as a basis ride (HgCl2) and analyzed in Hamburg at the Institut für Bio- for quick-look plots and printouts. After the failure of the first chemie und Lebensmittelchemie (U. Brockmann). Nominal ac- curacies are 0.7% for nitrate, 3% for phosphate, and 0.5% for dissolved silica. However, as the nutrient samples had not been filtered, there may be a systematic error due to absorption that 1 results in a linear reduction of the phosphate and dissolved sil- Rangin, C, Silver, E., von Breymann, M. T., et al., 1990. Proc. ODP, Init. ica values. Although a direct comparison with earlier data is Repts., 124: College Station, TX (Ocean Drilling Program). 2 Institut für Meereskunde, der Universitàt Hamburg, Troplowitzstrasse 7, therefore impossible, the relative structure of the profiles in the 2000 Hamburg 54, Federal Republic of Germany. Sonne data set is correct. 101 A. FRISCHE, D. QUADFASEL Table 1. List of hydrographic stations occupied during SO 58. Siαtion Dαiθ Time Latitude Longitude Depth (1968) Clocαlj [dbar] 19 28.6 7.30 - 7.58 13*55.18'N 119*53.51'E 1207 20 28.8 13.00 - 16.15 13°08.81'N 119*53.62'E 4656 21 28.8 19.10 - 21.20 12*55.65'N 120*22.67'E 2326 22 29.8 00.10 - 01.05 12*33.98'N 120*45.79'E 840 23 29.6 04.50 - 05.25 11*56.52'N 121*04.20'E 433 24 29.8 06.50 - 08.00 11*46.14'N 121*05.18'E 619 25 29.6 11.20 - 12.20 11*38.48'N 121°20.70'E 758 26 29.6 15.35 - 17.05 11*27.56'N 121*49.66'E 1077 27 29.8 19.10 - 20.15 11*14.76'N 121*57.85'E 838 28 29.8 21.50 - 23.05 ll*00.66'N 121*54.82'E 1215 34 31.6 00.15 - 01.40 10*29.90'N 121*45.10'E 1397 35 31.6 03.55 - 06.30 10*10.33lN 121*42.46'E 2912 40 1.9 03.10 - 06.00 9*50.75'N 121°37.17'E 3949 51 3.9 09.05 - 12.45 9*18.67'N 121*34.27 'E 4834 73 9.9 01.05 - 04.10 9*28.00'N 121*51.08'E 4143 77 10.9 02.33 - 05.55 8*45.60'N 121*41.00'E 4873 63 12.9 00.05 - 03.19 6*14.94'N 121*43.98'E 5064 106 17.9 11.15 - 11.59 7*12.21'N 121*46.41'E 454 107 17.9 21.12 - 00.51 7*12.08'N 121*06.14'E 4725 Altogether, 18 hydrographic profiles were obtained with a to- In the deep-water regime below about 2500 m depth, the ver- tal of 198 water samples. tical and horizontal gradients of the hydrographic properties are small. There is a tendency for the deep water in the northern DISCUSSION Sulu Sea to be slightly fresher and warmer and to have higher The vertical distribution of potential temperature, salinity, oxygen content compared to the deep water in the south (Fig. dissolved oxygen, and silicate content is shown in Figure 2. 2). The high temperature, low salinity, and high oxygen content Below the warm and fresh surface layer, the core of the Sub- are indicative of an intrusion of northern Pacific Intermediate tropical Lower Water from the Pacific Ocean can be seen around Water in the deep northern Sulu Sea. Deep plume convection 200 m depth as a very distinct salinity maximum. It lies well down the continental slope is a likely generation mechanism. above the sill depth in the Mindoro Strait and its spreading is However, due to its high temperature and low salinity, this thus not hampered by topography. plume water is less dense than the deep water in the central ba- North Pacific Intermediate Water, characterized by a salinity sin and therefore convection should not have occurred. and oxygen minimum at temperatures between 7.5° and 9.5°C, So far this apparent contradiction remains a puzzle. A possi- is clearly visible in the South China Sea at depths between 400 ble explanation is that the deep convection occurs in the form of and 800 m. This water is the primary source for the intermedi- turbidity currents, where the sediment load increases the density ate and deep waters of the Sulu Sea. A low-salinity core origi- of the sinking plume. However, this interpretation is very specu- nating from this minimum can be traced across the Sulu Sea in lative at this time and requires further investigation. the 600- to 1400-m depth layer. Is is associated with a relative The maximum depth of the renewal depends strongly on the maximum in oxygen content and a vertically homogeneous sili- density of the water inflowing through the Mindoro Strait (Fig. cate concentration. Temperatures here are higher than in the 4). The shallower the thermocline in the South China Sea, the corresponding layer in the South China Sea, presumably due denser the overflow water and the deeper the convection in the to tidally induced vertical mixing in the Mindoro Strait. The Sulu Sea. A possible mechanism for thermocline uplifting is the high oxygen concentrations (high compared with earlier data; passage of tropical cyclones. Two major storms passed the area Wyrtki, 1961; Broecker, et al., 1986) suggest that this intrusion during August 1987, causing catastrophic flooding in the Phil- of intermediate water occurred relatively recently. ippines. This event may have been responsible for the intermedi- The buildup of this intermediate layer and the effects of the ate depth intrusion in the Sulu Sea, as observed in August 1988. sills in the Mindoro Strait in cutting off the lower portion of the REFERENCES Pacific Intermediate Water is illustrated in Figure 3. Here verti- cal profiles of silicate content from the South China Sea, the Broecker, W. S., Patzert, W. C, Toggweiler, J. R., and Stuivr, M., 1986. Mindoro Strait, and the Sulu Sea are shown. The first sill off Hydrography, chemistry and radioisotopes in the southeast Asian Mindoro between stations 22 and 23 (470 m) limits the inflow to Waters.